1. Field of the Invention
This invention relates to a valve trim and more particularly to a low-noise valve trim.
2. Description of the Prior Art
The noise generated by a fluid flowing through a valve has many sources. The most significant are violent turbulent mixing at low pressure drops across the valve, and shock waves at high pressure drops across the valve. At high pressure drops, above the critical pressure ratio, shock waves and complex interaction between turbulent mixing zones and shock waves are the main sources of noise when the fluid is compressible, such as a gas. When the fluid is a liquid, high pressure drops can also cause cavitation, another major noise source.
Two common approaches exist for reducing valve noise: reducing the noise generated and shielding the valve body and downstream piping with sound absorbing material.
Shielding has been effective to some degree, but is generally inconvenient and inefficient. The sound absorbing material covers the valve body and downstream piping, requiring labor for installation and making maintenance to the valve costly. And, at best, shielding only "covers" the noise; it does not act on the source. And because noise also causes valve vibration, the valve damage that often accompanies noise is not eliminated by shielding.
Reducing the noise generated by the fluid flowing through the valve is, therefore, the more promising and common approach. In cage trim valves, in which a cage trim guides a valving element in its travel relative to a valve seat, the trim configuration has been a focus of noise-reducing efforts. The typical modification to the trim restricts the fluid flow rate through the trim, which reduces fluid velocity and thus reduces noise. The prior art shows many such modifications, for example, U.S. Pat. No. 2,918,087 to Curran; U.S. Pat. No. 3,602,261 to Brown, et al.; U.S. Pat. No. 3,693,659 to Parola; 3,780,767 to Borg, et al.; U.S. Pat. No. 3,813,079 to Baummann, et. al; and U.S. Pat. No. 4,041,982 to Lindner. All of those patents employ variations on a single approach: create myriad flow paths with small cross-sectional areas through the trim so that the fluid velocity through each flow path is reduced. However, those and all other known prior art devices for solving the problem of valve noise by reducing fluid velocity through the trim tend to become clogged by particulate matter entrained in the fluid and are difficult to manufacture. Thus, the known prior art approaches increase the cost of manufacturing and maintenance and decrease reliability.
In many fluid handling environments, for example, chemical or petrochemical plants, a valve that enhances mixing would be desirable. Many of the valve trims shown in the prior art inherently aid mixing and many devices have been proposed for fluid mixing, for example, U.S. Pat. No. 2,740,616 to Walden; U.S. Pat. No. 3,582,048 to Sarem; and U.S. Pat. No. 4,102,359 to Patel; and German Pat. No. 564,628 and British Pat. No. 166,542.
However, the known prior art does not include a valve trim that will reduce noise levels, efficiently mix the fluid flowing through the valve and resist clogging by particulate matter entrained in the fluid flow.